Since analog-digital conversion using a ramp wave can convert numbers of analog signals to digital signals in parallel, much attention has been paid to the conversion as means for increasing speed of a system such as an image sensor, in which large numbers of pixel signals need to be converted to digital signals.
In this technique, analog-digital conversion is performed by comparing an individual analog signal provided to each of comparators and a ramp wave provided commonly to the comparators. Thus, characteristics of the analog-digital conversion largely depend on characteristics of the ramp wave. Therefore, glitch noise, which is usually included in a ramp wave, needs to be removed by a low-pass filter, as described in, e.g., Patent Document 1.    PATENT DOCUMENT 1: Japanese Patent Publication No. 2008-299716    PATENT DOCUMENT 2: Japanese Patent Publication No. 2006-337139